DoD operates numerous stationary and mobile conventional demilitarization furnaces at various facilities. Air pollution regulations are becoming more stringent and require installations with these furnaces to control hazardous air pollutants (HAPs). The National Emission Standard for Hazardous Air Pollutants (NESHAP) for hazardous waste combustors for incinerators includes reduction of particulate material emissions from 0.013 grams per dry standard cubic foot (g/dscf) to 0.0015 g/dscf. Allowed lead and cadmium emissions have been reduced from 230 micrograms per dry standard cubic meter (μg/dscm) to 10 μg/dscm. Similarly, the arsenic, beryllium and chromium standard has been reduced from 92 μg/dscm to 23 μg/dscm.
Preliminary analysis of emissions from the U.S. Army's demilitarization furnaces, such as mobile Ammunition Peculiar Equipment (APE) 1408 Brass Certification Unit and stationary munitions deactivation furnace APE 1236, indicates the presence of lead, cadmium and other metals. The U.S. Army operates a number of these brass certification units at various locations. The development of air pollution control technology allows for the continued use of these units within regulatory constraints, significantly reducing release of HAPs.
The primary difficulty with controlling HAPs from deactivation furnaces derives from typical flow rates (e.g., greater than 500 cfm) of toxic metal vapors and particulates in emissions at temperatures reaching 550° C. Emissions at high flow rates require expensive pollution control systems. Speciation studies conducted by the Army Corps of Engineers, Engineer Research and Development Center (ERDC), Construction Engineering Research Laboratory (CERL), on emissions from deactivation furnaces, found that lead, cadmium, antimony and other metals are released in two phases: solid particulates and vapors. Nearly 97% of the metals are in particulate form. Thus, if the solids are captured in an initial treatment stage, solid phase metal emissions are significantly reduced. However, it is also necessary to capture vapor phase metal compounds to meet NESHAP standards.
A literature search revealed a few works dealing with the removal of lead vapor from a gas stream. Yang et al. reported a method of reducing volatile lead emissions from waste incineration by high temperature capture of vapor phase metals before they condense into fine particles. Packed bed sorption experiments with calcined kaolin at 973-1173° C. were conducted. Lead reacted with the sorbent to form water insoluble lead-mineral complexes. Increased bed temperature resulted in increased capture rates, but it had no effect on maximum uptake. Diffusional resistance developing in the interior of the porous kaolin particles became limiting only after the conversion of lead-kaolin reached a value greater than 50%. (Yang, Hee-Chul et al., Mechanism and Kinetics of Cadmium and Lead Capture by Calcined Kaoline at High Temperatures, Korean J. Chem. Eng., 18(4), 499-505, 2001).
Wronkowski reported adsorption of tetraethyl lead on two kinds of activated carbons at 18° C. with partial pressure from 0.03-0.9 atmosphere. The amount adsorbed depended on the specific surface of the given carbon and on the structure of its pores. (Wronkowski, Czeslaw, Adsorption of Tetraethyl Lead Vapors on Activated Carbon, Gaz. Woda Tech. Sanit., 39(4), 131-132, 1965).
Uberoi and Shadman evaluated several sorbents for removal of lead compounds, mainly PbCl2. The sorbents were silica, alpha-alumina, and any of the natural compounds including kaolinite, bauxite, emathlite, and lime. All experiments were conducted at 700° C. At this temperature PbCl2 chemically reacted with the sorbent producing both water soluble and insoluble compounds. The authors provided relative sorption capacity, with kaolinite giving the best result. (Uberoi, M. and Shadman, M., High-Temperature Adsorption of Lead Compounds on Solid Sorbents, AIChE Journal, Vol. 36 at 307-309, 1990).
Wey and his coworkers studied the adsorption mechanisms of heavy metals, including lead, on silica sands using a fluidized bed system operated from 600-800° C. At this temperature range chemical reactions, rather than a physical adsorption, are preferred. They noted that for lead, both chemical and physical adsorption mechanisms are important and depend on the reacting environment. Saturation adsorption capacities of silica sand for lead were 16.08 mg/g at 600° C. and 12 mg/g at 800° C. (Chen, Scott et al., An Evaluation of Carbon-Based Processes for Combined Hg/SO2/NOx Removal from Coal Combustion Flue Gases, Book of Abstracts 216th ACS National Meeting, Boston, Aug. 23-27, 1998. Chen, J-C. et al., Adsorption Mechanism of Heavy Metals on Sorbents During Incineration, J. of Environmental Engineering, Vol. 127, No. 1, pp. 63-69, 2001).